1. Field of the Invention
The present invention relates to a light-emitting element including an electroluminescence (also referred to as EL) layer (the element also refereed to as an EL element), a light-emitting device including the light-emitting element, and an electronic device and a lighting device each including the light-emitting device.
2. Description of the Related Art
An EL element includes a pair of electrodes and an EL layer provided therebetween. A voltage is applied to the pair of electrodes, so that light is emitted from the EL layer. The EL layer is formed using an organic compound.
Light emission mechanism of the EL element will be described. Upon applying voltage to the pair of electrodes, electrons injected from the cathode and holes injected from the anode are recombined in a light-emission center of the EL layer. As a result of the recombination, energy is released so that light is emitted. Light emission is classified into emission from a singlet excited state and emission from a triplet excited state.
Light-emitting elements have various problems, one of which is how to reduce the driving voltage.
According to Patent Document 1, an organic compound of an electron injection layer in contact with a cathode is doped with a metal having a low work function such as an alkali metal, an alkaline earth metal, or a rare earth metal. Such a doping with the metal having a low work function lowers the barrier against electron injection from the cathode to the electron injection layer, thereby reducing the driving voltage.
According to Patent Document 2, optical adjustment of an emission spectrum can be performed without increasing driving voltage. Specifically, a layer in which an organic compound having a hole-transport property is doped with a mental oxide is provided between a cathode and an EL layer of a light-emitting element so as to be in contact with the cathode. Further, so as to be in contact with the metal oxide-doped layer, a layer in which an organic compound having an electron-transport property is doped with a metal having a low work function such as an alkali metal, an alkaline earth metal, or a rare earth metal is provided. The thickness of the metal oxide-doped layer is adjusted so that an increase of the driving voltage is suppressed and optical adjustment of emission is performed.